A charge coupled device (CCD) imager typically employed as an image scanner contains an array of light detecting photosites (hereafter imaging pixels) which accumulate charge depending on the light energy projected onto them. After some charge accumulation time, the charges in the imaging pixels are transferred to a charge shifting structure so that the charges may be shifted out of the CCD and measured by a signal processing circuit in order to form an image signal representative of the image projected onto the CCD. Because of manufacturing variability in the CCD, dust or contaminants in the optical path which projects an image onto the CCD, light source non-uniformity, or other source of variation, the system response for individual imaging pixels may not be the same from pixel to pixel. Compensation for this pixel-to-pixel variation may be provided in the charge measuring process. This compensation can be provided by multiplying the output value for each site by a gain value and then adding an offset value. This pixel-by-pixel application of gain and offset makes the responsiveness of all the sites appear to be equal.
Typically, the system response for a given imaging pixel does not change in the short term. Hence, the gain and offset values required to adjust the system response for a given imaging pixel back to some ideal response can be determined by a calibration process and then applied whenever the signal for that imaging pixel is shifted out of the CCD. A typical calibration process obtains samples of the system response for each imaging pixel at some nominal signal input level (a white, gray, or black card, or full illumination or dark, for example) at some nominal gain and offset values (typically 1 and 0, respectively) and then calculates the required gain and offset values for each of the imaging pixels. However, previously described calibration processes require that the gain and offset mechanisms provide known responses. To the extent that component variations or other factors introduce error, these calibration processes will not be correct. This disclosure describes a calibration process which is insensitive to variations in the gain and offset mechanisms.
Site by site compensation for variations in imaging pixel response is well known in the art. U.S. Pat. No. 3,800,079--McNeil et al describes a system whereby a "sensitivity profile" of all the light detecting sites is created by scanning a "standard background or white level which may be in the form of a document or a bedplate." This profile is converted to digital values and stored in a digital memory for subsequent readout and conversion to an analog compensation signal during an operational mode. The uncompensated video is divided by the analog compensation signal, thereby providing gain correction. Correction for offset variations is also described. This employs storage for a second profile which is obtained by scanning black. This patent lays the foundation for the concept of scanning white to determine gain compensation required, scanning black to determine offset compensation required, and storing these compensation values for readout during an operational mode.
As a typical example, Tomohisa et al. (U.S. Pat. No. 4,660,082) exhibits the shortcomings usually found in the calibration methods for a system employing imaging pixel by imaging pixel application of gain and offset compensation signals. As is usual, this patent describes a system where two sets of reference values are collected, one set by scanning a white reference board and the other set by scanning a black reference board. It is suggested in the patent that "the two density reference voltages are preferably obtained by scanning the corresponding density reference boards with the gain and offset values of the output amplifier at `1` and `0` respectively." However, because of manufacturing tolerances or drift due to temperature variations or aging of components, these desired `1` and `0` levels may be slightly off, leading to error in the resulting compensation values, or requiring an iterative approach to determining the compensation values. In general, the methods described in the prior art all depend on some expectation that some reference values always be set accurately to known levels and/or A/D and D/A conversion mechanisms have known conversion constants and offsets which, in general, is an ideal situation that is virtually impossible to realize in practice. It is desirable, therefore, to provide a method for determining compensation values for gain and offset in an imager signal processing circuit which is insensitive to the internal characteristics of the signal processing chain.